An Ace Hardware big rig plunged from a highway overpass in Vallejo on Thursday morning and burst into flames, killing its 44-year-old driver and forcing the temporary closure of portions of two highways, authorities said.

Around 6:45 a.m., the truck was westbound on Highway 37 when it hit a pickup truck and swerved off an overpass above Highway 29, said Officer Kevin Bartlett of the California Highway Patrol. The big rig fell from a height of about 30 feet, landed on its side and caught fire.

Witnesses who had been driving on Highway 29, including an off-duty paramedic, ran to the truck and found the driver hanging upside down, tangled in his seat belt, said CHP Officer Chris Parker. They pulled the driver out of the truck and he was taken to a hospital, where he died around 7:10 a.m.

He was identified as William Ballard of Citrus Heights (Sacramento County). The pickup truck driver was not injured.

The CHP shut down westbound Highway 37 between Interstate 80 and Highway 121 for nearly three hours before reopening it about 9:30. Northbound Highway 29 was shut down until noon.

Firefighters let the truck burn because they were unsure of what was inside, Parker said. The Solano County hazardous materials team also responded to the scene.

The cause of the crash is under investigation.

Ace Hardware said that the company was "deeply saddened" by Ballard's death. It did not say how long he had worked for the company.